A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
U.S. Pat. No. 6,333,775, which is incorporated herein by reference, describes a lithographic apparatus, including multiple pressure zones for controlling particle contamination on the reticle and carbon contamination of optical surfaces of optical surfaces. The known system includes a vacuum apparatus that includes various compartments that are maintained at different pressures. One of the compartments is a reticle zone which encases a reticle stage. Below the reticle zone, an optics zone is provided which encases projection optics devices. A reticle metrology tray separates the reticle zone from the optics zone. The reticle zone is maintained at a vacuum pressure of less than about 100 mTorr and preferably at about 30 mTorr. The optics zone is maintained at a vacuum pressure of less than about 5 mTorr. A seal assembly provides a gas limiting seal at the outer perimeter of the reticle metrology tray. The center of the reticle metrology tray includes an aperture through which light beams enter and exit. During use, about 200 L/s (litres per second) gas flows through this aperture from the reticle zone into the optics zone.
In the known apparatus, the reticle stage is kept in a reticle zone, partially separated from the projection optics. Therefore, the volume of the optics zone can be relatively small, including a relatively small number of components, compared to an apparatus wherein the reticle stage and the projection optics are located in the same vacuum environment. Consequently, the optics zone can be pumped down to a desired vacuum level relatively fast.
A disadvantage of the known lithography apparatus is that a relatively large amount of contamination can still reach the optics zone from the reticle zone via the aperture. Such contamination, for example, small particles, hydrocarbon and/or water, may hamper the operation of the optics, as well as spoil the optics as such. This hinders a respective device manufacturing method, leading to relatively expensive and/or low-quality devices manufactured thereby.